Since electronic apparatuses have been downsized and the functionalities thereof have been advanced, the high densities of printed wiring boards having the various electronic devices mounted thereon have been demanded. In response to these demands, multi-layered printed wiring boards (multilayer printed wiring boards) have been developed.
To achieve the high density, for example, a hybrid multilayer circuit board has been developed (Patent Literature 1). The hybrid multilayer circuit board includes two multilayer circuit boards (hard circuit boards) and a flexible printed wiring board (or a flexible flat cable) that connects the multilayer circuit boards.
The above multilayer printed wiring boards and the hybrid multilayer circuit board have been widely used for small electronic devices such as laptop computers, digital cameras, portable communication devices, and game machines.
In such electronic devices, the signal transmission speed tends to become higher and higher because the amount of information handled by the electric devices particularly increases. For example, in personal computers, the transmission standard shifted to a standard with the transmission rate of 6 Gbps during 2010 to 2011. The importance of considering the signal loss in transmission lines is increasing.
Conventionally, a transmission line in a printed wiring board that transfers signals at a high speed is matched with a characteristic impedance. Accordingly, unnecessary reflection at respective boundaries among a signal driver, the transmission line, and a signal receiver is prevented, thereby preventing the deterioration in signal quality and securing the transmission characteristics. For example, a single end transmission line is designed generally so as to have a characteristic impedance of 50Ω.
When the resistive component in a transmission line is neglected, a characteristic impedance is expressed by Formula 1.
                    [                  Expression          ⁢                                          ⁢          1                ]                                                                      Z          0                =                              L            C                                              (        1        )            
In the above formula, Z0 is a characteristic impedance, L is an inductance, and C is a capacitance.
As is seen from Formula 1, the value of the characteristic impedance is defined not only by physical properties such as the dielectric constant of the insulating layer, the conductivity of the wiring conductor, the permeability or the like in the printed wiring board but also by physical shapes such as the width and thickness of the transmission line (a signal line), the distance (that is, the thickness of the insulating layer) between the signal line and the ground layer, and the like.